Motor control circuit



Jan. 25, 1949. CURTIS 2,460,064

MOTOR CONTROL CIRCUIT Filed May 16, 1945 AT TORNEY Patented Jan. 25, 1949 MOTOR CONTROL CIRCUIT Austen M. Curtis, South Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 16, 1945, Serial No. 594,063

16 Claims. 1

This invention relates to an amplifier circuit and more particularly to a motor control circuit having a vacuum tube amplifier stage, a relay stage and a motor stage.

The simplest relay control circuit which will permit a motor to be driven in either direction and at a speed dependent upon a weak control signal, employs two relays which are vibrated continuously by an external alternating current source of power at a periodicity of 20 to 60 cycles per second. The circuits through the motor over the contacts of the relay are such that when the relays in vibrating close their swing contacts to their front and back contacts for equal periods, no power is suppliedto the armature of the motor if the motor is of the direct current type or to the control phase winding if the motor is of the two-phase alternating current type.

The percentage of time during which the swing contact of each relay is in its front or back contact closure position is controlled by a direct current through a second winding on the relay. This current serves both as a bias and as a signal and is preferably derived from a stage of tube amplification interposed between the signal input circuit and the relays. The direction of rotation of the motor is then dependent upon whether the signal or control current is above 01' below the normal bias value which controls the polarity of-the power applied by the relays to the motor and the strength of the signal or control current above or below the normal bias value determines the duration of the impulses of power and thus the speed to which the motor will attain. A motor control circuit of this type is disclosed in the application of J. T. L. Brown and C. E. Pollard, Jr., Serial No. 594,048, filed May 16, 1945.

While this simple circuit gives a satisfactory performance for small motors it does not operate as well with larger motors. Part of the difficulty is that the relays found best adapted for use in such circuits, that is relays of the mercury contact reed armature type which are disclosed in the application of E. T. Burton, Serial No. 545,985 filed July 21, 1944, operate with a single contact transfer in which all three contacts are joined together for one or more milliseconds during each operation. Under some circumstances this bridging period may last longer particularly if the relays have been in service for a long time or become damaged. The current which the contacts will handle during these transfer periods without being damaged is about 12 amperes at 28 volts. This leads to the circuit arrangements in which the bridging of all three relay contacts will short-circuit the power supply and it is necessary to protect against this by an inductance which prevents a significant current from being drawn during the normal brief bridging and also by a resistance which limits the maximum current during a long flash and prevented damage to the contacts. This necessary protection limits the power which can be handled to about 30 watts at 28 volts direct current. The maximum length of impulses which can be applied to the armature of the motor during stable controlled operation of both relays is about equal to a quarter of the drive period. During the inactive part of the relay cycle of oper ation the motor armature is short-circuited and a large proportion of the applied power is wasted in heating the armature of the motor.

The performance of this and all other direct current motor control circuits is improved by inserting between the relay contacts and the motor armature an inductance comparable in value to the inductance of the motor which stores energy during the applied power period of the operation of the relays and returns it to the armature during the braking period. If properly proportioned to the motor, load and drive frequency this prevents dynamic braking of the motor itself during the brief periods when the relays are centered and much improves the efficiency and smoothness of the torque developed by the motor.

But even with the added armature circuit inductance the operation of the balanced relay circult is not as desirable fora large motor as the circuit which has'been developed in accordance with the present invention. If the two relays do not match'each other rather closely in dynamic operating characteristics there are likely to be inequalities in the curve of power versus control current and the cont-act protection problems become more difficult.

It is the object of the present invention to improve a motor control circuit of the type just described whereby motors of larger power may be controlled over the contacts of the control relays in response to a control or input signal, whereby the requirement that the relays should be substantially matched in their operation response is obviated and whereby impulses of a character best adapted for the most efficient mode of operation are delivered to the relays which control the motor.

In accordance with the present invention the relays may be caused to operate and release in alternation by a sine Wave current applied to the exciting windings of the relays from an external source having a frequency from 20 to cycles per second or the relays may be operated in by current applied to the exciting windings from multivibrator tubes.

If the first method of operation is used it is found convenient to precede the relays with a pair of triode tubes and to transfer the control function of the relays to the tubes. The relays will now function in accordance with the sine wave exciting current as modified by the biasing current applied to their control windings under the control of the triode tubes which in turn are under the controlof' the input signal; pulses of power are nowtransmitted directlyto the motor under the control of these relays the wave form of the impulses will not be the best for efficient motor operation and sparking will occur at the contacts of the relay. I have-found,

however, that if these relaysare-usedmerely. as control relays to control the operation of-"an. additional pair of drive relayswhich in turn serve to apply drive impulses to the motor then the drive relays will receive simplecurrent impulses of controlled length and of a waveform more suitable for good'operation of the drive relays than are the current impulses from..the. triode tube plate circuits andalso, the motor will then receive drive impulses of a WEWB, form better suited for the operation thereof.

In carrying out this feature of the inventionthe plate circuitsof the triode tubes are supplied with a direct .current voltage of about. 180'volts on"v which is superimposed an alternating, current voltage of about .120. volts and ata frequency from 20 to,l0 .cycles per. second. The direct cur-'- rent control or signal. voltage is applied to the grids of the tubes. With the circuit'properly adjusted for balance andwith no control voltage both-the control and drive relays are vibratedby short impulses and; the driverelaysbeingvibrated in alternation, the pulses transmitted to the motor armature alternate in phase thereby failing-to cause the motorto .rotatebut with sum-- cient. vibration. to reduce. its inherent static fric- With .acontrol voltage-applied to the grids tion. of thetubes so that one grid is made more; negative with respect to the associated cathode then.

at. a-low control the tube having the negative gridpotential becomes blocked-and the relayat that sideceases to-vibraterand therefore only one side of"the'circuit'operates.

Instead of an external source of alternating current the relay vibrating frequency may be supplied by'vacuum tubes.

employs as anoscillating 'elementani unbalanced multivibrator the output impulses from each:

multivibrator operating a 'triode tube whichfunm tions to operate one of the drive relays. The multivibrators, with no control input; give output waves ofabout 40 cycles per second frequency and the impulses which reach the drive relays are too short to be effective to cause the relays to operate. When a control'signal is applied to the multivibrators, the impulses to one associated drive relay are increased in lengthand the im pulses applied .tothe, otherw drive relay are decreasedlin length causing the. relayonthe strong; side. of the circuitv to transmit. driving impulses to the motor. The impulse length increasesin proportiontothe control voltage until the drive relay onthe strongfside. of the circuit locks up and the motor then runs at full speed. Suitable couplingis provided between-the two multivibrators to insure that theywill operate at the same frequency when the control signal-isweak or en-.- tirely absent.

On-e difficulty which has beenfoundwith'the multivibrator circuit is that the relay on the weak side of .the'circuit'inight-not stop vibrating-unless a very carefuladjustment ofeach multivibrator isfmade sothat'withno control .voltagesthe relays. willnot operate. from. their back: contacts other wise the relay on the strong side when locked up could not apply full'power to. the motor'and'the.

If im- In accordance with this feature of the invention each side of. the circuit weak side relay would tend to vibrate and chop upthe impulses; Also this conditioncauses some flashing of therela-y contacts. When. thev multivibrators are so adjusted that this condition I. could not possibly happen the width of this part around zero control was made undesirably wide. Toovercomethis difficulty it is proposed as a furtherieature: ofthe present invention to crossconnect thewindings of the drive relays into the plate. circuits'o'fithecontrol tubes so that the high trolwinding of. the" latter relay to thereby stop it from vibrating. This arrangement is also effective .in-a-circuit in-whichthe control relays are vibrated from an: external source of alternating current.

For a clearer understanding of: theinvention referencemaybe hadtothefollowing detailed description when read'in connection with the accompanying drawing, in which? Fig. 1 shows 5a motorfcontrokcircuit:in accord ance. with; the: present invention: in which: bOth'.

control and drive": relays r are" employed and in which the alternatingcurrent excitation of the control, relays. is provided? from an; external source; and" Fig. Z'shows a" modified: form: of a motor control circuit in which only drive relays are employed whichare vibratable under the controloi signalcontrolledmultivibrator tubes.

The relays illustrated in the drawing are preferably of the reed armature-mercury contact type disclosed'in the application of'Ei T. Burton hereinbeiore referred to. It is to be understood however that relays of other types could be used if such relays are sensitive andiiast in operation and are capable of handlingcurrent oi considerable wattage transmitted over their contacts. Each of these relays comprisesan" envelope of. glass or other suitable material through the bot tom of which an armature't'erminal iss-eale'd and through-the top of whichltwo otherterrninals are sealed. Secured to-thelinnerendofone oithe upperterminals isa-front contactofmagnetic ma? terial and securedto the. inner end'otthe other upper terminal is a bachcontact. or non-ma neticmateriaL. securedtozthe lower terminal by a reed is. armature. of magnetic material which is. normally .biased against; the back contact and is attractabletowards the front contact. A pool of mercury isplacedinthebottom of the envelope from which'mercury isconducted by wick actionto the-upper contact portions. Sui rounding the envelope are-:tWO-"SOiIs or windings, one of which is of-high resistance; for example, of 3300 ohmsvand the'other of which is of lower resistance, iorexampleofflOO ohms.

Referring first::to 'Fig. llwhich shows one embodimentiof theinvention; azdirect current control signalis appliedtover: terminals 3 and t to the: control grids of the dualztriodei tube. VT. The control. signal appliedi betweensthe. terminals 7 3 and 4 may for example be derived from the output circuits of amplifier tubes which proceed the tube VT. The grids 5 and 6 of tube VT are also connected through equal grid leak resistors I and 8 to ground. The plate 9 of tube VT is connected in circuit through the high resistance winding H of control relay CA, thence through the low resistance winding L of control relay CB shunted by resistance I2 and condenser 24, through the secondary winding SA of transformer T and through rheostat I8 to a source of plate potential connected to terminal I3, which source may, for example, be of 180 volts positive potential. Simi larly, the plate I of tube VT is connected in a circuit through the high resistance winding H of control Ce, thence through the low resistance winding L of control relay Cs shunted by resistance II and condenser 25, through the secondary winding SB of transformer T and through rheostat I8 to the source of plate potential connected to terminal I3. By the adjustment of the slider of rheostat I8 the inequalities in the two halves of the tube VT may be substantially compensated by the adjustment of the plate potential applied to the two plates. The cathodes I4 and I of tube VT are connected together and through condenser I6 to ground and through rheostat I9 to ground, the rheostat I9 serving to adjust the cathode to grid potentials. The filament I! of the tube is heated by current conducted therethrough, through the winding of rheostat I9 and through the rheostat from the terminals I and 2 of the 24-volt direct current source P. The filament current may be adjusted by the rheostat 20.

The motor M is of the shunt field type the field F of which is excited from the 24-volt power source P connected to terminals I and 2 and the armature circuit A of which is connected between the swing contacts or armatures of the drive relays DA and DB. The back contacts of relays DA and DB are connected to the terminal I of source P and the front contact of eachof these relays is connected through an inductance coil L1 and thence through the inductance coil L3 shunted by resistance 2| to the other terminal 2 of the source P. The inductance coil L3 and resistance 2I serve to protect the power source P from becoming short-circuited during the intervals of one or more milliseconds when the three transfer contacts of either drive relay will be interconnected. For contact protection a resistance R1 connected in series with condenser C1 is bridged between the swing and back contact of each drive relay, a resistance R2 connected in series with condenser C2 is bridged between the swing and front contact of eachdrive relay and inductance coils L1 and L2 connected in series with condenser C3 are also bridged between the swing and front contacts of each relay. The contact protection is essentially the same as disclosed in the application of E. T. Burton, hereinbefore referred to.

For smoothing the operation of the motor M in response to power impulses transmitted through the armature circuit A under the control of relays DA' and DB the inductance coil L4 shunted by resistance 22' is connected in series with the armature circuit. The inductance of the coil L4 is comparable in value to the inductance of the motor and stores energy during the applied power periods of the operation of the drive relays and returns it to the armature circuit during the periods when the relays do not apply power to the armature circuit.

The swing contacts of control relays CA and CB are both connected to terminal 2 of power source P. The front contact of rela CA is connected to the H and L windings of drive relay DA connected in parallel, and-through the rheostat 23 to the terminal I of the source P and similarly the front contact of relay CB is connected to the H and L windings of drive relay DB connected in parallel, and through the rheostat 23 to the terminal I of source P whereby when relay CA operates, it in turn causes the operation of relay DA into its front contact closure position and when relay CB operates it in turn causes the op eration of relay DB into its front contact closure position.

The primary windings of transformer T are connected in series and supplied with alternating current from the source AC which, for example, may be of volts potential and 60 cycles per second frequency.

With no signal present and therefore no signal potential applied between terminals 3 and 4 and with the biases of tube VT adjusted so that both units of tube VT are substantially equally con ductive, current will flow from the positive volt source through rheostat I3, secondary winding SA of transformer T, through the L winding of relay Cs in parallel with resistance I2, through the H winding of relay CA and over the platecathode path through the upper unit of tube VT to ground through rheostat I9; and also through rheostat I8, through the secondary winding SB of transformer T, through the L winding of relay CA in parallel with the resistance I I, through the H winding of relay CB and over the plate-cathode path through the lower unit of tube VT to ground through rheostat I9. The H and L windings of relays CA and CB are connected in opposition so that the direct biasing currents flowing through the two windings of each relay tend to oppose each other but, under the zero signal condition assumed, the current through the H winding being much greater than the current through the L winding of each relay, each of the relays will be so biased as to be operable during each positive half Wave of the alternating current from the source AC applied through their H windings.

The alternating current from the source AC of 60 cycles is impressed through the windings PA and SA of transformer T through condenser 24, which is connected in shunt of the L winding of relay CB and resistance I2, thence through the H winding of relay CA, over the plate-cathode path through the upper unit of tube VT and through rheostat !3 to ground and is similarly impressed through the windings PB and SB of transformer T, through condenser 25, which is connected in shunt of the L winding of relay Ca. and resistance II, thence through the H winding relay CB, over the plate-cathode path through the lower unit of tube VT and. through rheostat I9 to ground.

Due to the poling of the secondary windings of transformer T the alternating current components of the currents flowing through the H windings of relays CA and CB will be 189 degrees apart in phase and consequently relays CA and C13 in response to the superposed direct current biasing and the alternatin current exciting currents will operate and release in alternation at the pericdicity determined by the periodicity of the alter nating current source AC.

Each time that relay CA operates to its front contact closure position a circuit is established from terminal I of the power source P, over the upper portion of rheostat 23 through the wind- 'tinued energization of drive relay D will be been applied in the opposite sense to the input terminals 3 and 4 then as the signal strength increased from zero the direct current component of the current applied through the lower unit of tube VT would have increased and the direct current component of the current applied through the upper unit of tube VT would have decreased resulting in the operation of relays CA and CB, and DA and DB in such a manner as to cause the motor to run in the reverse direction and at a speed commensurate with the magnitude of the input signal.

From the foregoing discussion of the circuit of Fig. 1 it is apparent that at zero signal input the control relays CA and CB operate alternately and in turn cause the alternate operation of the drive relays DA and DB so that the motor does not turn but merely vibrates to reduce its static friction. That as the signal increases from zero in either direction both control relays CA and CB operate and the unbalance between the length of the impulses transmitted to the motor by the operation of the drive relays DA and DB increases so that the motor will gradually pick up speed as the signal strength increases and turn in a direction dependent upon the polarity of the signal. That as the input signal further increases the control relay on the weak side of the circuit will cease operating and the entire control will then be exercised by the control relay on the strong side of the circuit. By this arrangement the range on either side of zero control is made much narrower and linearity of the entire range is improved. Furthermore the control of the motor is improved by the interpolation of the control relays CA and CB. If relays CA and CB were used to drive the motor directly the slow rate of change of the drive and control flux of the relays CA and CB near the point at which they lock up would contribute to the flashing of the contacts of such relays, especially as in this regien the contact pressure is practically. nil. The addition of the relays CA and CB helps to improve this condition since now the relays CA and CB are used as control rather than drive relays and supply square or nearly square waves to the drive relays DA and DB which gains something over the just operate flux even on the shortest impulse.

The stopping of one of the control relays and the corresponding drive relay when the value of the control signal is increased from the zero value materially increases the life of the relays since most of the time only two of the four relays will be functioning.

If an external source of alternating current for exciting the control windings of the relays is not available, the relays may be operated periodically by impulses of current supplied under the control of multivibrator tubes as has been disclosed in Fig. 2. For this purpose a pair of tubes interconnected to act as a pulse generator of a substantially square-topped wave, is used in each side of the circuit, the output of one pair of such tubes being applied to the input of one unit of a buffer tube, the plate circuit of which is connected through the H Winding of one of the drive relays BA and the output of the other pair of tubes being connected to the input of the other unit of the bufier tube, the plate circuit of which is connected through the H winding of the other drive relay DB. Each multivibrator functions in the general mannerdescribed onpage 294 of the 10 book by A. V. Eastman entitled Fundamentals of Vacuum Tubes.

More specifically the multivator tube MVA has its cathodes 28 and 2! connected together and to ground,-has its control grid 28 connected through grid leak resistances 30 and 3| to ground and has its control grid 29 connected through grid leak resistance 32 to ground. The plate 33 of the upper unit of this tube is connected through the resistance 51, of approximately 5100 ohms, and through the right portion of rheostat 36 to the positive terminal of plate battery B1 and the plate 34 of the lower unit of this tube is connected through the left portion of rheostat 36 to the positive terminal of battery B1 and thus upon the adjustment of the rheostat 36 unbalanced plate potentials are applied to the plates of this tube. For producing an oscillatory condition between the two units of this tube the plate 33 of the upper unit is connected through condenser 31 to the grid 29 of the lower unit and the plate 34 of the lower unit is connected through condenser 38 to the grid 28 of the upper unit. The condenser 3'! and 38 are of equal capacitance, preferably .03 microfarad, and with resistance 3! substantially short-circuited over the control circuit connected between terminals 3 and 4, the discharge paths for the condensers are equal or substantially so through the equal resistances 3B and 32.

Similarly the cathodes 39 and 40 of tube MVB are connected together and to ground, the grid M is connected through resistances 43 and 44 to ground and the grid 42 is connected through resistance 45 to ground. The plate 46 of the lower unit of the tube is connected through resistance 53 and through the right portion of rheostat 48 to the positive terminal of the plate battery B1 and. the plate 41 of the upper unit of the tube is connected through the left portion of rheostat 48 to the positive terminal of battery Bl and thus upon the adjustment of the rheostat 48 unbalanced plate potentials are applied to the plates of the tube. For producing an oscillatory condition between the two units of the tube the plate 46 of the lower unit is connected through condenser 49 to the grid 42 of the upper unit and the plate 4'! of the upper unit is connected through condenser 50 to the grid of the lower unit. The condensers 49 and 50 are of equal capacitance. preferably .03 microfarad, and with resistance 44 substantially short-circuited over the control circuit connected between terminals 3 and the dischar e paths for the condensers are equal or substantially so through the resistances 43 and 45.

Since unequal plate potentials are applied to the plates 33 and 34 of tube MVA the condenser 31 and 33 will receive unequal charges due to the firing of the units of the tube and the tube will therefore generate an unbalanced. output wave. Similarly with unbalanced plate potential-s applied to the plates 46 and 41 of tube MVB the condensers 48 and 59 will receive unequal charges due to the firing of the units of the tube and the tubewill therefore generate an unbalanced output wave. Since the two tubes are tied to the same plate battery, then with the proper adjustment of the rheostats 36 and 48, the two tubes will generate output waves which will tie together at the same frequency when no control signal or a weak signal is applied between the input terminals 3 and 5 which are in turn connected respectively to the junction point between resistances 3i! and 3! and the junction point between resistances 43 and. 44.

The plate 33 of the tube MVA is connected 11 throu h, resistance 59, or approxi te 150,000 ohms, to the grid of the upper unit of the buffer tube BT and the plate of the tube MVB is similarly connected through resistance 59 to the grid 5 2 of the lower unit of tube ET. The resistances 59 and 6t serve to eliminate the cheats of the output of the buifer tube BT upon the sensitivity of the multivibrator tube to control signals. The cathodes 53 and 54 of tube BT are both connected to the negative terminal of the plate battery B2 and to the positive terminal of battery B1. The filaments of all of the tubes may beheated in any well-known manner from a suitable source of current as, for example, from the ,2fi-volt power source P. The plate 55 of the upper unit of tube ET i connected through the H winding of drive relay DA, the L winding of drive relay De shunted by resistance 12 and condenser 68 and through the lower portion of the rheostat 65. to the positive termina1 of battery B2 and the plate 56 of the lower unit of tube ET is connectedthrough the H winding of drive relay DB, the L winding of drive relay DA shunted by resistance H and condenser 61 and through the upper portion of rheostat 65 to the positive terminal of battery B2. By the adjustment of the slider of rheostat 85 a balanced output from the two units of tube BT may be secured when there is no control signal applied between the terminals 31 and Al.

The high resistance winding H of drive relay DA is shunted by a .02 microfarad condenser '6! connected in series with a 1000-ohm resistance E3 to eliminate the induced voltage when the tube BT cuts oft the plate current in its upper unit and to make the relay operate into its front contact closure position on slightly shorter impulses than it would without the shunt by using the stored energy to continue to accelerate the relay armature. A similar shunt comprising the condenser 62 and resistance 64 is connected around the H winding of relay De. The low resistance winding L of the relays are connected in parallel with resistances H. and I2 and condensers 61 and 68, respectively and are connected in opposition to the H windings of thecorresponding relay. The armature circuit A and the motor M is connected to the armatures or swing contacts of the relays and the same contact, battery and motor protection is provided as is disclosed in Fig. 1 and discussed in connection therewith.

With no control signal present between the terminals 3 and. ,4 the inultivibrator tubes MVA and MVB will generate output wavesof the same periodicity, the output pulses of which will be tied together occurring either simultaneously or nearly so. Each impulse from the multivibrator tube MVA is impressed upon the upper unit; of bufier tube BT and in response thereto plate current flows from the positive terminal of the plate battery 132 over the lower portion of rheostat 65, through the L winding of relay DB in parallel with resistance 12 and condenser 58 through the H winding of relay DA and over the plate-cathode path through the upper unit of tube BT' to the negative terminal of battery B2. Each impulse from the multivibrator MV is impressed upon the lower unit of tube BT and in response thereto plate current flows from the positive terminal of plate battery B 2, over the upper portionof rheostat 65, through the L winding of relay DA in parallel with resistance I l, through the H winding of relay DB, and over the platecathode path through the lower unit of tube BT to the negative terminal of; battery Ba, With the QAQQQQQQ 1 2 H and w ndin s of" eac relay. connected. in opposition the current flowing through theH and Ir w nd n s each elay will set up pp s n ilows oi; flux and with the current impulses delivered, by the tube BT under the. control, of the m ltivibrator tubes. MVA and MVB short, neither relay will be sufiiciently energized to cause its swing contact to. move intoits front contact closure positionand consequently no circuit will be, established from the source of power P through the armature circuit A of the motor M.

It will now be assumed that a negative signal voltage isv applied to the input terminal 3 and a positive signal voltage of equal value is applied to the input terminal 4 as for exam lefrom the center tapped battery B3 under control of the rheostat 6,6. Asa result the multivibrator tube MVA is. caused to. generate longer impulses. In responsetothe longer impulses generated by the tube MVA, relay DA now operates in response to each impulse butsince the impulses generated by the tube MVB have-been shortened relay DB does not operate. Asa result oftheoperation of relay DA- while relay DB remains unoperated, impulses of one polarity are transmitted from the source P through the armature circuit A and the motor M and the motor'will attain a speed proportional to the length of the impulses delivered to the relay DA and therefore proportional to the control signal volt-age.

To insure that the short impulses delivered by the multivibrator tube MVB will not cause relay DB to vibrate its swing contact; away from its back contact during each interval that relay DA is operated to cause the transmission of a driving impulseof the motor; and thus cause the driving impulses; to be chopped-up, the operating current applied through the H winding of relay DB in responseto the impulses generated by the m-ulti vibrator tube MVB, is opposed by the increasing direct current component of the current delivered through the L winding of relay DB in response to the impulses generated; by the multivibrator tube MVA which is delivering operating impulses for causing the operationoi relay. DA. The current flowing through the L winding of relay- 13B .under this condition so unbalances the current flowing through the Hwinding that the swing contact of relay ,DB is held firmly in its back contact closure position.

As the signal voltage further increases the length of the impulses generated by the multivibrator tubefMV 'increases an cl as a; result the relay DA will deliver impulses of increasing length tothe motorlvi thereby causing thespeed of the motor to increase until when the control voltage as applied to the grid 28 of multivibrator' tube MVAreachesa maximum negative magnitude the upper unit of tube MVA ceases to fire and the multivibrator ceases to generate impulses. At this time the upper unit" of buffer-tube BT will conduct continuously thereby locking drive relay DA in its front contact closure position to cause themotor M to run continuously at its full speed.

As the positive signal potential applied to the grid 41 of inultivibrator tube MVB increases, the lowerunit of this tube will lock-up in a conducting condition and" the multivibrator will then cease-to generate further impulses. This conditio-nwillbe reached soonv after the input signal increases to a low value from zero voltage.

If a'negative signal voltage is applied to the input terminal 74- and a corresponding positive voltage is applied to the terminal 3, the impulses generatedi by the multivibrator tube MVB will increase in length as the signal voltage increases and as a consequence the impulses transmitted to the drive relay DB will increase in length and at the same time the drive relay DA will be prevented irom operating. Impulses will therefore be transmitted from the source P through the armature circuit of the motor M in such a direction as to cause the motor to run in the opposite direction and at a speed proportional to the voltage of the control signal. Relay DA will be locked in the back contact closure position and multivibrator tube MVA will be controlled to cease transmitting pulses as the signal increases from its zero value in the manner previously described in connection with the operation of tube MVB and relay DB and tube MVB will reach a condition in which it ceases to generate impulses and relay DB will lock in its front contact closure position to cause motor M to run. at full speed when the signal voltage reaches its maximum value in the .manner previously described in connection with the operation of tube MVA and relay DA,

The control of the drive relays by the multivibrators enables the relays to operate on substantially square-topped impulses thereby improving the motor operation without the use of additional control relays as was necessary in the circuit of Fig. 1. Since one of the drive relays is blocked against operation either through the cross-connections between the windings of the two relays or by the blocking of the multivibrator tube on the weak or positive signal voltage side of the input circuit, wear on. the relays is reduced by half and steadier impulses are delivered to the motor. Also by blocking the multivibrator tube in the strong or negative voltage side of the input circuit at maximum signal voltage and the consequent locking of the drive relay on that side of the circuit the wear on the relays is further reduced. Both modifications of the circuit enable a much smoother operation of the motor adjacent to the zero control and improves linearity of control over a greater portion of the control range.

What is claimed is:

1. In a motor control circuit, two control tubes, a relay associated with each of said tubes each relay having a high resistance and a low resistance winding connected in opposition, an energizing circuit for each relay including the oathode-plate path of the associated tube, the high resistance winding of such relay and the low resistance winding of the other relay, means for transmitting impulses of current over said circuits, a source of power, a reversible motor, means controlled by said relays for applying power from said source to said motor, and an input circuit for selectively controlling said tubes to apply current to one of the other of said first circuits, whereby one of said tubes is eifective to render the associated relay operable and is efiective to energize the low resistance winding of the relay associated with the other tube to render the latter relay unresponsive.

2. In a motor control circuit, two control tubes, a relay associated with each of said tubes, each relay having a high resistance and a low resistance winding connected in opposition, an energizing circuit for each relay including-the cathode-plate path of the associated tube, the high resistance winding of such relay and the low resistance winding of the other relay, means for transmitting impulses of current over said circuits, a source of power, a reversible motor, circuits controlled when either of said relays is operated and the other of said relays is unoperated for applying power from said source to said motor, and an input circuit for selectively controlling said tubes to apply current to one or the other of said first circuits whereby one of said tubes is effective to render the associated relay operable and is effective to energize the low resistance winding of the relay associated with the other tube to maintain said latter relay firmly in its back contact closure position.

3. In a motor control circuit, two control tubes, a relay associated with each of said tubes, each having a high resistance winding and a low resistance winding connected in opposition, an energizing circuit for each relay including the cathode-plate path of the associated tube, the high resistance winding of such relay and the low resistance Winding of the other relay, means for applying direct and superimposed alternating currents to said circuits, a source of power, a reversible motor, means controlled by said relays for applying power from said source to said motor, and an input circuit for selectively controlling said tubes for varying the direct current applied through one or the other of said first circuits whereby the direct current component of the plate current of one of said tubes is efiective to render the associated relay operable and is effective to energize the low resistance winding of the relay associated with the other tube to render the latter relay unresponsive.

4. In a motor control circuit, two control tubes, a relay associated with each of said tubes, each having a high resistance winding and a low resistance winding connected in opposition, an energizing circuit for each relay including the cathode-plate path of the associated tube, the high resistance winding of said relay and the low resistance winding of the other relay, means for applying direct and superimposed alternating currents to said circuits, a source of power, a reversible motor, circuits controllable when either of said relays is operated and the other of said relays is unoperated for applying power from said source to said motor, and an input circuit for selectively controlling said tubes for varying the direct current applied to one or the other of said first circuits whereby the direct current component of the plate current of one of said tubes is effective to render the associated relay operable and is effective to energize the low resistance winding of the relay associated with the other tube to maintain said latter relay firmly in its back contact closure position.

5. In a motor control circuit, two control tubes, 8. control relay associated with each of said tubes, an energizing circuit for each of said relays including the cathode-plate path of the associated tube, means for applying direct and superimposed alternating currents to said circuits, a motor drive relay operable under the control of each of said control relays respectively, a source of power, a reversible motor, circuits controllable when either of said drive relays is operated and the other of said drive relays is unoperated for applying power from said source to said motor, and an input circuit for selectively controlling said tubes for varying the direct current applied to one or the other of said control relay energizing circuits whereby the direct current component of the plate current of one of said tubes isefiective to render the associated control relay operable to in turn controltherassociated drive relay to cause said motorto operate. at a speed operable under the control of eachof saidconl trol relays respectively, a source o'f-power, aireversible motor, circuits controlled when either of said drive relays is operated and theother of said drive relays is unoperated for applying power from said source to said motor, and an--inputcircuit for selectively controlling said tubes ior varying the direct current applied to oneor the other of said control relay energizing circuits whereby the direct current componentof the plate current of one of said tubes is efiective to render the associated control relay operable and is efiective to energize the low resistance winding of the control relay associated with the other tube to maintain said latter relay firmly in its back contact closure position to in turn control the associated drive relay to cause said motor to operate at a speed determined by the control exercised over said input circuit.

7. In a motor control circuit, two control relays, a vacuum tube multivibrator associated with each of said relays, input circuits connected to the inputs of said multivibrators, means for applying a control signal to one or the other'of said input circuits whereby the associated multivibrator is controlled to deliver impulses-tothe associated relay of a length proportional to the magnitude. of the signal, a source of power, a reversible motor and circuits controllable bysaid relays for applying impulses of power fromsaid source to said motor whereby saidmotor is operated at a speed proportional to the magnitude of the signal and in a direction determined. by the manner of application of the; signal.-

8. In a motor control circuit, two control relays, a vacuum tube, multivibrator associated with each of said relays, input circuits connected to the inputs of said multivibrators, means for applying a control signal to either one .of saidinput circuits and a signal of opposite polarity'and of the same magnitude to the other of said in-' put circuits whereby one of said multivi'brators is controlled to deliver impulses to the associated relay of a length proportional to the magnitude of the signal and whereby the other of said multivibrators is controlled tov deliver impulses to-the associated relay which decrease in length as'th'e magnitude of the signal increases, a source of power, a reversible motor, and circuits controlled by said relays for applying impulses of power from: said source to said motor whereby said motor is operated. at a speed proportional to the magnitude of the signal and in a direction de-- termined by the direction of application of the signal.

' 9. In a motor control circuit, two control tubes, a control relay associated with each of said tubes, an energizing circuit for each of said relays including the cathode-plate pathof the associated tube and a source of direct current, a vacuum tube multivibrator associated with the input circuit of each of said tubes, input circuits conwhereby said motor is operated at a speed proportional to the magnitude of the signal and in a direction determined by the manner of application of the signal.

10. :Inja motor control circuit, two control tubes, :"a control relay associated with each of said tubes,:an energizing circuit for each of said relays including the cathode-plate path of the associated tube and a source of direct current, a vacuum tube multivibrator associated with the input circuit of each of said tubes, input circuits connected to the inputs of said multivibr'ators, means for applying a control signal to eitherone of said input circuits and a signal of opposite polarity and of the same magnitude to the other of said input circuits whereby one of said multivibrators controls the associated tube to deliver'impulses to the associated relay which increase in length as the magnitude of the signal increases and whereby the other of said multivibratorscontrols the tube associated therewith todeliver impulses to the associated relay which decrease in length as the magnitude oi-the signal increases, a source of power, a reversible motor, and circuits controllable by said relayssfor applying impulses of power from said source to said motor whereby said motor is operated at a speed proportional to the magnitude of the signal and in a direction determined by the directionof application of the signal.

11. In a motor control circuit, two control tubes, a control relay associated with each of said tubes, an energizing circuit for each of said relays including the cathode-plate path of the associated tube and a source of direct current, a vacuumitube multivibrator associated with the input circuitof'each'of said tubes, means for unbalancing said multivibrators whereby they normally cause the associated tubes to deliver impulses to-said relays of insuflicient length to cause theiroperation, input circuits connected to the inputs :of said multivibrators, means for applying-a control signal to one or the other of said inputrircnits whereby the associated multivibrator controlsthe associated control tube to cle liver impulsesof sufficient length to operate the relayassociated therewith for impulse intervals proportionalato the magnitude of the signal, a sourcerofpower, a reversible motor and circuits controllable when eitherof said relays is operatedsand-the other of said relays in unoperated for applying impulses of power from said source to sai-d motor whereby said motor is operated at a speed proportional to the magnitude of the signal;

12. In a motor control circuit, two control tubes, a control relay associated with each of said tubes,'an energizing circuit for each of said relays including the cathode-plate path of the associated tube and a source of direct current, a vacuum tube multivibrator associated with the input circuit of each of said tubes, means for unbalancing said multivibrators whereby they normally cause the associated tubes to deliver impulses to said relays of insufficient length to cause their operation, input circuits connected to the inputs of said multivibrators, means for applying a control signal to either one of said input circuits and a signal of opposite polarity and of the same magnitude to the other of said input circuits whereby one of said multivibrators controls the associated tube to deliver impulses to the associated relay which increase in length as the magnitude of the signal-increases and of suiiicient length to operate said relay for intervals proportional to the magnitude of the signal and whereby the other of said multivibrators controls the tube associated therewith to deliver impulses to the associated relay which decrease in length as the magnitude of the signal increases, a source of power, a reversible motor, and circuits controllable by said relays for applying impulses of power from said source to said motor whereby said motor is operated at a speed proportional to the magnitude of the signal and in a direction determine by the direction of application of the signal.

13. In a motor control circuit, two control relays, a vacuum tube multivibrator associated with each of said relays, means for unbalancing said multivibrators whereby they normally cause the delivery of impulses to said relays of insufiicient length to cause their operation, input circuits connected to the inputs of said multivibrators, means for applying a control signal to either one of said input circuits and a signal of opposite polarity and of the same magnitude to the other of said input circuits whereby one of said multivibrators is controlled to deliver impulses to the associated relay of a length proportional to the magnitude of the signal and whereby the other multivibrator is controlled to deliver impulses to the associated relay which decrease in length as the magnitude of the input signal increases from zero over the lower portion of its range and is controlled to cease delivering impulses to the associated relay as the input signal further increases. a source of power, a reversible motor, and circuits controllable by said relays for applying impulses of power from said source to said motor whereby said motor is operated at a speed proportional to the magnitude of the signal and in a direction determined by the direction of application of the signal.

14. In a motor control circuit, two control relays, a vacuum tube multivibrator associated with each of said relays, means for unbalancing said multivibrators whereby they normally cause the delivery of impulses to said relays of insufiicient length to cause their operation, input circuits connected to the inputs of said multivibrators, means for applying a control signal to either one of said input circuits and a signal of opposite polarity and of the samemagnitude to the other of said input circuits whereby one of said multivibrators is controlled to deliver impulses to the associated relay of a length proportional to the magnitude of the signal and whereby the other multivibrator is controlled to deliver impulses to the associated relay which decrease in length as the magnitude of the signal increases, said first multivibrator locking up to cause the transmission of a continuous impulse to the associated relay when said signal approaches its maximum magnitude, a source of power, a reversible motor, and circuits controllable by said relays for applying impulses of power from said source to said motor whereby said motor is operated at a sleeve 18 proportional to the magnitude of the signal and in a direction determined by the direction of application of the signal.

15. In a motor control circuit, two control relays, each having a high resistance and a low resistance winding connected in opposition, an energizing circuit for each relay including a source of current, a high resistance winding of such relay and a low resistance winding of the other relay, a vacuum tube multivibrator associated with each of said circuits, a source of power, a reversible motor, circuits controllable when either of said relays is operated and the other of said relays is unoperated for applying power from said source to said motor, input circuits connected to the inputs of said multivibrators, and means for applying a control signal to either one of said input circuits and a signal of opposite polarity and of the same magnitude to the other of said input circuits whereby one of said multivibrators is controlled to deliver impulses to the energizing circuit of the associated relay which increase in length as the magnitude of the signal increases and whereby the other of said multivibrators is controlled to deliver impulses to the energizing circuit of the associated relays which decrease in length as the magnitude of the signal increases and whereby the impulses delivered through the opposing windings of the relay on the weak side of the circuit are effective tomaintain' said latter relay firmly in its back contact closure position.

16. In a motor control circuit, two control tubes, a control relay associated with each of said tubes, each having a high and a low resistance winding connected in opposition, an energizing circuit for each relay including the cathode-plate path of the associated tube, the high resistance winding of such relay and the low resistance winding of the other relay, a vacuum tube multivibrator associated with the input circuit of each of said tubes, means for unbalancing said multivibrators whereby they normally cause the associated tubes to deliver impulses to said relays of insufficient length to cause their operation, a source of power, a reversible motor, circuits controllable when either of said relays is operated and the other of said relays is unoperated for applying power from said source to said motor, input circuits connected to the inputs of said multivibrators, and means for applying a control signal to either one of said input circuits and a signal of opposite polarity and of the same magnitude to the other or said input circuits whereby one of said multivibrators controls the associated tube to deliver impulses to the energizing circuit of the associated relay which increase in length as the magnitude of the signal increases, whereby the other of said multivibrators controls the associated tube to deliver impulses to the energizing circuit of the associated relay which decrease in length as the magnitude of the signal increases and whereby the impulses delivered through the opposing windings of the relay on the weak side of the circuit are effective to maintain said latter relay firmly in its back contact closure position.

AUSTEN M. CURTIS.

REFERENCES CITED UNITED STATES PATENTS Number Name Date Whitman Jan. 9, 1934 

